President Biden has expressed concern that a dispute over the Northern Ireland Protocol - the Brexit deal that prevents checks along the Irish border - could threaten the peace process.
He's thought to have raised the subject during face-to-face talks with Boris Johnson, ahead of this weekend's G7 summit.
What is the Northern Ireland Protocol and why is it needed?
During Brexit negotiations, all sides agreed that protecting the Northern Ireland peace deal (the Good Friday agreement) was an absolute priority.
When that was signed in 1998, one of the key issues was the need for an open border between the Republic of Ireland and Northern Ireland.
That was easy to settle because both were part of the European Union. The Republic and Northern Ireland shared the same EU rules on trade, so no checks were needed on goods travelling from one country to another.
However, a new arrangement was needed after Brexit. The EU requires many goods to be inspected when they arrive from non-EU countries, while some products aren't allowed to enter at all.
The EU and the UK had to come up with a new way of ensuing there were no border controls. This was why they negotiated and signed up to the Northern Ireland Protocol.
It came into force on 1 January 2021, and is now part of international law.
How does it work?
Under the protocol, both sides agreed that, even though Northern Ireland was no longer part of the EU, it would continue to follow many of its rules.
This would enable lorries to continue driving across the land border without being inspected.
Meanwhile, England, Scotland and Wales are no longer following those rules - leading to a new "regulatory" border between Great Britain and Northern Ireland.
New checks on goods now need to be carried out when they enter Northern Ireland from England, Scotland or Wales.
Inspections take place at Northern Ireland ports, and customs documents have to be filled in.
This has prompted criticism that a new border has effectively been created in the Irish Sea.
What problems have arisen?
The new system got off to a shaky start, with some disruption at the beginning of the year.
Some food products arriving in Northern Ireland from Great Britain - such as frozen meat, milk, fish and eggs - have to be monitored to ensure they meet EU standards.
They need to go through a border control post, where paperwork is checked and some physical inspections take place.
The EU said in early February that the control posts were not yet fully operational and some goods were entering Northern Ireland without being properly declared.
To make sure supplies were maintained, supermarkets were allowed to avoid having their products checked for three months.
In March, the UK decided - by itself - to extend this grace period until October. It subsequently announced further unilateral moves, to make the trade in parcels and plants from GB to Northern Ireland easier.
The EU has said the UK's decision to extend the grace period breaks international law, because it wasn't consulted. It has launched legal action which could end up with the European Court of Justice imposing substantial fines on the UK.
On a visit to Northern Ireland on 12 March, before the EU legal action began, Boris Johnson insisted the government's move was lawful.
What's the row about sausages?
EU food safety rules don't allow chilled meat products to enter its market from non-members, such as the UK.
This has led to the prospect of British sausages being banned from Northern Ireland.
A six-month grace period (separate to the supermarket grace period) has been in place since January where the rules don't apply.
But under the terms of the protocol, it runs out at the end of this month.
Talks on what happens next were held between the UK and EU earlier this week, but they broke up without agreement.
Maros Sefcovic - a vice-president of the European Commission - said he was "positive we can find a solution" but added that the EU's patience was "wearing very, very thin".
If an agreement can't be reached, the UK says it is ready to ignore the rules. However, the EU has warned it will respond with legal action which could lead to the introduction of tariffs (taxes on imports) on British goods.
What about security concerns?
As well as problems with trade, there are also political and security concerns.
Checks were temporarily suspended at the beginning of February, over what were described as "sinister" threats to some border staff checking goods.
Unionists are strongly opposed to the checks because they don't want Northern Ireland to be treated differently to the rest of the UK. In March, one group wrote to the Prime Minister to withdraw support for the Good Friday agreement.
There have also been a series of demonstrations and protests against the idea of any kind of border in the Irish Sea.
Speaking after his meeting with President Biden on 10 June, Boris Johnson said that there was "complete harmony on the need to keep going, find solutions and make sure we uphold the Belfast Good Friday Agreement".